pyroclastic flow

Pyroclastic flow sweeps down the side of Mayon Volcano,
Philippines, during an explosive eruption on 15 September 1984. Note
the ground-hugging cloud of ash (lower left) that is billowing from
the pyroclastic flow and the eruption column rising from the top of
the volcano. Credit: C. Newhall / U.S. Geological Survey

A pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic
gas that rushes down the side of a volcano as fast as 100 km/h or more. The temperature within a pyroclastic flow may
be greater than 500°C, sufficient to burn and carbonize wood. Once deposited,
the ash, pumice, and rock fragments may deform (flatten) and weld together
because of the intense heat and the weight of the overlying material.